Regulated membrane fusion is essential to many cell processes and to the life cycle of lipid-sheathed viruses such as HIV, Influenza, and Ebola. Several proteins ("fusion machines") involved in neurotransmitter release and enveloped viral infection have been identified and characterized structurally. Still, how these proteins catalyze fusion is not fully understood. The Lentz lab studies lipid rearrangements associated with fusion between synthetic membranes. The approach is first to define these rearrangements in pure lipid systems and then to ask how fusion proteins might promote them. Fusion requires close contact between membranes, which is induced using the inert polymer poly(ethylene glycol) (PEG). Fusion between lipid vesicles aggregated by PEG is shown to be minimally a three-step process (contacted bilayers D "stalk" intermediate D "diaphragm" intermediate D pore) that mimics biomembrane fusion. The Lentz group calculated, using the mechanical properties of lamellar lipid phases, the free energy reaction profile of this "stalk" fusion mechanism and showed it to be consistent with their unique studies effusion kinetics. Experiments and calculations show that the free energies of bent lipid monolayers and of defects between non-lamellar and lamellar structures (hydrophobic interstices) dominate the fusion process. Most researchers have focused on the bending energy to explain fusion. The Central Hypothesis is that fusion proteins catalyze fuSion not just by altering bending energy but in good measure by stabilizing hydrophobic interstices. To test this hypothesis, the project will address seven Specific Aims: 1] Compare the abilities of different bend-inducing lipids to partition into or promote bent lipid structures, and 2] alter the fusion reaction mechanism;3] Determine whether an infection-blocking mutation in the membrane spanning domain of HIV alters the structure of a synthetic membrane spanning domain peptide, alters membrane structure, and alters the effect of this peptide on fusion;4] Determine whether mutations in a key region of Influenza virus (the "fusion" peptide) alter a} the structure of a synthetic fusion peptide, b} membrane structure, and c} the effect of this peptide on fusion;5] Determine whether neurotransmitter- release-blocking mutations in the membrane-spanning region of a neuronal fusion protein (syntaxin) alter a} the structure of a synthetic membrane spanning domain peptide, b} membrane structure, and c} the effect of this peptide on fusion;6] Determine by electron microscopy whether fusion pores form at the point of contact between membranes held in contact by neuronal fusion proteins;and 7] Determine the ability of a neuronal calcium-binding protein (synaptotagmin) to perturb, and trigger fusion between, model membranes brought